The present invention relates to a method and apparatus for monitoring haemodynamic function in humans and animals and, particularly, but not exclusively, to a method and apparatus for monitoring haemodynamic function in humans and animals during anaesthesia and surgery, and its relationship to anaesthetic depth.
During anaesthesia and surgery on a human or animal subject, the subjects haemodynamic respiratory, neuromuscular and neurological functions are monitored as indicators of the condition of the health of the subject as anaesthesia and surgery progress. In general, as anaesthetic (depth) increases, haemodynamic, respiratory and neurological function are depressed or decrease (ie. there is a dose-dependent relationship). During any operation, it is important that adequate perfusion is maintained (ie. oxygenated blood reaches all vital organs including the brain, heart and kidneys). Tissue oxygen delivery is dependent on the level of perfusion or blood flow (cardiac output [CO]) and the amount of oxygen in the arterial blood (Arterial Oxygen Content, CaO2). Haemodynamic function (causing blood flow to vital organs) is therefore carefully monitored and any changes which indicate that haemodynamic function may not be optimum will alert the anaesthetist who may adjust the anaesthetic does to compensate ie., to vary the depth of anaesthesia by adjusting anaesthetic depth.
Traditional monitoring of haemodynamic function in anaesthetised patients undergoing surgery, in particular humans, is based on cardiac auscultation, an ECG (electro cardiogram) and blood pressure measurement. Cardiac auscultation will detect the rate of heart beats. The ECG directly monitors cardiac rhythm (electrical rhythm of the heart) and indirectly monitors the pulse rate (assuming the electrical rhythm causes an organised heart muscle contraction). Blood pressure monitoring devices measure blood pressure, usually measure pulse rate and the information obtained is used by clinicians/anaesthetists to indirectly make inference about (estimate) haemodynamic function, i.e., cardiac output (total blood flow) and organ perfusion. The pulse rate, cardiac rhythm, blood pressure, and inference about haemodynamic functions provide the information necessary to give the anaesthetist an overall picture of haemodynamic function during anaesthesia and surgery.
This type of traditional monitoring of haemodynamic function, in particular the use of blood pressure monitors, is subject to a number of problems.
Indirect blood pressure monitors (systems using a pneumatic cuff and a method to detect the arterial pulse) are inaccurate in small animals, horses and human infants and automated devices can be expensive. Direct blood pressure monitors (systems using a catheter placed in an artery, connected to a pressure measuring device) are accurate but invasive, complex and expensive. Catheterisation of an artery is also NOT done without some risk of complication to the patient.
Further, the general perception in anaesthesia has been that good blood pressure equals good haemodynamic function. This is, if the blood pressure is good, it is taken as an indication that there is adequate blood flow to ensure perfusion of all the vital organs. During anaesthesia and surgery good blood pressure together with good results for the other indicators (cardiac rhythm, pulse rate, etc) has generally been taken to mean that everything is going well for the patient.
The majority of anaesthetic agents depress cardiac output in a dose dependent fashion. Generally, therefore, low blood pressure has been taken to indicate that anaesthetic dose should be lightened and high blood pressure that anaesthetic does should be increased (although the other indicators also have a bearing on anaesthetic does and the anaesthetist will take all indicators into account before deciding on the appropriate action).
The present applicants have realised that blood pressure is not in fact as good an estimator of cardiac output or perfusion during anaesthesia and surgery as has traditionally been considered. Firstly, indirect measurement of blood pressure is inaccurate and secondly it is, in fact, frequently negatively related to total blood flow (cardiac output) and tissue oxygen delivery.
There is a recognised relationship between blood pressure, cardiac output and vascular resistance, as follows:Cardiac Output =Blood Pressure (Map−Right Artrial Press)+Vascular Resistance.
One major problem with the usual assumption that blood pressure gives an indication of cardiac output is that none of the usual clinical measurements (auscultation, electrocardiogram, blood pressure) provide any information about vascular resistance.
During surgical procedures at usual anaesthetic levels, it is believed that the subject body may still experience and respond to painful stimulation, although the subject is not consciously aware of the pain. The body, however, produces its standard sympathetic nervous system response to the painful stimuli, including catecholamine release, resulting in vasoconstriction. The applicants believe that such responses lead to increases in blood pressure during surgery being accompanied by a decrease in cardiac output. This is exactly opposite to the relationship between blood pressure and cardiac output which clinical anaesthetists have traditionally assumed. During painful surgery, therefore, rather than a direct positive relationship between blood pressure and blood flow there is believed to be a variable relationship which may even be in a negative direction.
Given the above observation, and also the fact that non-invasive blood pressure monitors are inherently inaccurate, it is clear that, in anaesthetised patients undergoing surgery, blood pressure cannot be relied on as an accurate estimator of haemodynamic function.